stickypatch

So, I now have Hankook K110 tyres on every corner of my new-to-me 1994 C2 - 255/40/17 rear / 205/50/17 front. I have the pressures at 44 rear and 36 front - as the sticker on the door jamb says. I don't find the ride too hard as I have been driving stiff cars for a while now and it seems pretty normal.

I'm a very average pedaller - I haven't yet had any time on a track in any car, or time with an instructor - but I am looking forward to it as part of joining the NZ POC which holds regular DE days as my insurance covers me for this use (higher excess if I ding the car, of course). I do drive in a spirited manner now and again - but I'm very aware of my limits and that my last three cars (all Audis) had stability control that (might have) saved my bacon if I really mucked things up - but that this Porsche does not have stability control or any driver aids other than ABS.

I've searched tyre pressure threads on the board, and I can see that the consensus is that 44R/36F is too hard - especially for the rears. So, my question is why? What are the consequences? Does having 44 psi at the rear result in less traction or higher wear? Different and less desireable handling qualities? How does one go about methodically finding the right pressure for them?

The benefits of others' experience are much appreciated and I am grateful to everybody for their contributions.

cmat

For reasons that I still do not understand, the factory recommended pressures are 36F 44R for 16/18" and 36F 36R for 17". Is 17" the original size wheels on your car?

I have reduced the cold pressures in my 18" tires to 34F 38R and find it makes for a less harsh ride in my street driven car. I think many are running similar pressures with 18" wheels. Not sure what is best for 17".

Matt Lane

Yes, as above, the factory spec pressures on 17 inch wheels and tires of that size are 36/36. That will give you a much better ride - try it.

The 18's are spec'd at 36/44 - and the rear is way too hard. I run 38 or so.

Best,

Matt

nile13

The issue with overinflating your tires is that you are not using the tire's shoulder in turns. I will not go into effects and explanations here, let's just say it's not great.

The tire pressure suggestions by manufacturer should be ignored anyway. They are suggestions for very specific tires. When you buy aftermarket tires you should figure out what these particular tires like. K110 are not sold in US so it's hard to tell. If they are similar to V112 I'd say start with 32/36 and go from there depending on what you feel and what you want out of your tires and your car.

Matt Lane

Fair enough, yes, you can optimize pressures for good wear/performance. This may yield small diferences on street pressures.

But in the case of the OP and just regular street driving, 36/36 will yield good performance and wear as a starting point. The lower you go, the higher risk of puncture or bent rims. Plus less margin for cold weather or normal air loss over time.

nile13

Matt, I would disagree that going lower in pressure would increase risk of bent rims. I'm also not sure why puncture risk would increase.

I currently run about 30/34 on Dunlop Direzza Star Specs on the street and was playing with slightly lower pressures at autocross today (the car feels like a pig, which is to be expected by someone used to 2,080 lb Miata on Hoosiers). i pay no mind to recommendations that jump from 36/36 to 36/44. Too general, though I'm sure there's an explanation as to why Porsche did that.

I do feel that significantly heavier rear end of a 993 should generally benefit from slightly higher rear tire pressure. Than again, this would heavily depend on tire, alignment, springs, etc. With my two 993s the way the way they were both set up I end up with about 4 psi more pressure in the rear tires than the fronts.

NC TRACKRAT

Using the prescribed air pressure with stock suspension, most car manufacturers opt for slight understeer characteristics for safety's sake.
In general, increasing rear tire pressure will decrease understeer. However, it should be increased in small increments and with caution, especially if you're not familiar with the "tail wagging the dog"!

Ed Hughes

This is why Porsche has the reco pressures it does- to keep the car more prone to understeer. Decreasing the rear pressure will reduce this tendency, allowing the rear to scoot around a bit, making it less "idiot proof".

nile13

Ed, it doesn't really explain the huge difference they advocate between 16/18th and 17th. While I know that sidewalls are different on those, but that much difference just makes no sense to me.

Ed Hughes

I was focused on the 44/36 on the 17's. They used to reco 36/32 on the 911's 16's. I ran more like 32/30or 34/32 on the street, or lower even, at the track.

IXLR8

The "experts" published 36/36 for 17" wheels in the Owner's Manual.

vaujot

and
are saying the opposite of each other. Which is in line with other statements on the subject I have heard, i.e. there seems to be disagreement on the handling effects of increasing tire pressure on one end of the car vs. the other. I wonder what is the right answer.

NC TRACKRAT

I stand by my original post. Increasing rear tire pressure decreases understeer...or, if you will, increases oversteer. For the front tires, the opposite is true.

relinuca

But why does Porsche recommend significantly higher pressures for 18" rear wheels than 17"s? That is the question.

relinuca

Cactus

34/35F - 36/37 rear. This combo works very well for me.